Geophysical prospecting has been used extensively in the search for underground resources such as oil, gas, and minerals. Common techniques used for exploration include seismic, gravity, magnetic, and electrical methods. Seismic is historically the most widely used and can be subcategorized into seismic reflection and seismic refraction methods. With the seismic reflection method, the structure of subsurface formations is mapped by measuring the times required for a seismic wave, generated in the earth by a near-surface explosion, mechanical impact, vibration, or air gun, for example, to return to the surface after reflection from interfaces between formations having different physical properties. The reflections are recorded by detecting instruments responsive to ground motion or pressure waves. With reflection methods, one can locate and map, for example, such features as anticlines, faults, salt domes, and reefs.
The recorded data generally are processed using computers prior to being interpreted. The basic objective of seismic processing is to convert the information recorded in the field into a form that best facilitates geological interpretation. The field data are transformed into corrected record sections. One object of the processing is to eliminate or reduce noise. Another is to present the reflections with the greatest possible resolution.
Seismic sources may be placed in a wellbore is to provide seismic data that originates closer to the target formations of interest to enhance imaging and measurement accuracy and resolution. Applications include crosswell seismic, single-well seismic imaging, reverse vertical seismic profiling, and microseismic monitoring of velocity measurements, velocity calibration, and sensor orientation. In each of those methods, fine spatial sampling is required to meet the Nyquist criterion for spatial sampling and to provide a spatial multiplicity of measurements to enhance the accuracy of tomographic inversion methods and velocity calibration methods.